1. Field of the Invention
The present invention relates to a 1×N optical switch, an integrated optical switch including optical waveguide switches coupled in a multi-stage. The present invention further relates to an optical module that electrically controls its output characteristics and is applicable to an optical communication system, to an optical switch and an optical matrix switch constituting the optical module.
2. Description of the Related Art
To cope with a sharp increase in traffic volume in data communication networks typified by the Internet recently, a large increase in capacity is being carried out by using optical transmission technology such as wavelength division multiplexing (WDM) transmission. Recently, the optical transmission technology is only applied to point-to-point optical links interconnecting nodes, but not applied to the processing within each node which is still carried out electrically. As the large transmission capacity more increases, the electrical processing poses problems of a slow increase in throughput and a sharp increase in cost. An optical cross-connect system and optical add drop system using optical switches can implement cut-through process by handling almost all the optical signals within a node optically, thereby being able to dramatically increase the throughput with reducing the cost. Thus, the optical switch is an essential device for constructing a large capacity, flexible communication network at a lower cost.
The optical switches are implemented in various types. Above all, optical waveguide switches are excellent in mass productivity and miniaturization. Conventionally, optical waveguides have been fabricated from a variety of materials. For example, silica-based optical waveguides formed on silicon substrates are characterized by a low loss, high stability and good matching with optical fibers. In addition, many different varieties of optical components typified by arrayed waveguide grating (AWG) multiplexer/demultiplexers are actually used.
A 1×N optical switch, one of the integrated optical switches, is an optical switch enabling one input port to be connected to any one of output ports. For example, in a selecting switch for a monitoring device, a selecting switch from light sources, or an optical cross-connect system, it is applicable to important optical switches such as an N×N optical matrix switch consisting of a combination of a plurality of 1×N optical switches. As for silica-based waveguides, a 1×N optical switch has been implemented in a 1×128 scale.
FIG. 1 shows an arrangement of a conventional tree type 1×8 optical switch. It is implemented by connecting two 2×2 optical switch units to outputs of a 2×2 optical switch unit connected to an optical input port, and successively cascading 2×2 optical switch units in a 3-stage constitution. The 2×2 optical switch unit at an a-stage, b-row position is represented by a-b (e.g. 3-2). The third stage 2×2 optical switch units have their output waveguides connected to optical output ports via gate optical switches. The c-th gate optical switch from the top is denoted by G-c (e.g. G-3). Each 2×2 optical switch unit can connect selectively one of the two inputs to any one of the two output waveguides. Thus connecting the 2×2 optical switch units in cascade with multi-stage enables the whole to function as a1×N optical switch. The gate optical switches connected to the optical output ports improve the extinction ratio by carrying out ON/OFF operation.
FIG. 2 shows an arrangement of a conventional tap type 1×8 optical switch. A 2×2 optical switch unit connected to an optical input port has its first output waveguide connected to an input waveguide of the next-stage 2×2 optical switch unit, and its second output waveguide connected to an input waveguide of a gate optical switch connected to an optical output port. Thus, eight 2×2 optical switch units are connected in an 8-stage constitution. Although FIG. 1 shows an example of the tree type arrangement, a combination of the tree type construction and tap type construction is also possible. The tree type construction is superior in reduction in size and loss of the switch circuit. On the other hand, the tap type construction is superior in lower consumption of the switch circuit. For example, arrangements of the optical switches making use of these features of each type are disclosed in T. Goh, et al., “Large-scale integrated silica-base thermo-optic switches”, NTT Review, Vol. 13, No. 5, pp. 18–25, 2001.
FIGS. 3A–3C shows an arrangement of a conventional 2×2 optical switch unit. FIG. 3A is a plan view of a 2×2 optical switch unit 1100 built on silica-based waveguides. FIG. 3B is a cross-sectional view taken along the line A–A′ of FIG. 3A, and FIG. 3C is a cross-sectional view taken along the line B–B′ of FIG. 3A. The 2×2 optical switch unit 1100 is a Mach-Zehnder interferometer (called “MZI” from now on) type 2×2 optical switch including two arm waveguides 1103a and 1103b. The two arm waveguides include a thermooptic phase shifter utilizing thin-film heaters 1101a and 1101b, and have their both ends connected with 3 dB couplers 1102a and 1102b. 
An MZI type 2×2 optical switch having two arm waveguides 1103a and 1103b of an equal length is called a symmetric type MZI. In contrast, an MZI type 2×2 optical switch having two arm waveguides 1103a and 1103b with an optical path difference of half wavelength is called an asymmetric type MZI. According to a known interference principle, the symmetric type MZI propagates light along the crossed path (port 1A→port 2B) when the thermooptic phase shifter is not driven, and along the bar path (port 1A→port 2A) when the thermooptic phase shifter is driven because of the optical path difference of half wavelength caused by the thermooptic effect.
In addition, the optical path is continuously shifted from the crossed path to the bar path by continuously varying the optical path difference between the two arm waveguides from zero to a half wavelength by controlling the driving current supplied to the thin-film heaters 1101a and 1101b. In other words, the MZI type 2×2 optical switch operates not only as an ON/OFF switch, but also as a continuously adjustable analog switch between transmission and interruption of light. Accordingly, the MZI type 2×2 optical switch can be used as an attenuator or an optical branching circuit for carrying out multicast or broadcast by adjusting a distribution ratio between the crossed path and the bar path.
The gate optical switches as shown in FIGS. 1 and 2 use asymmetric type MZIs. The asymmetric type MZIs propagate light along the bar path (port 1A→port 2A) when the thermooptic phase shifter is not driven, but along the crossed path (port 1A→port 2B) when the thermooptic phase shifter is driven because the optical path difference of half wavelength is canceled out by the thermooptic effect. Thus, as for the gate optical switches, using the asymmetric type MZI makes it possible to economize the power consumption, and to take full advantage of the cross port with a higher extinction ratio.
Thermooptic switches using the silica-based waveguides are fabricated by combining a glass film deposition technique such as flame hydrolysis deposition (FHD) or chemical vapor deposition (CVD), with a micro etching technique such as a reactive ion etching method (RIE). More specifically, a glass film of an under cladding layer is formed on a substrate such as a silicon wafer, followed by depositing a core layer with a refractive index slightly higher than that of the cladding layer. Subsequently, a core pattern is formed by a micro etching technique, followed by depositing a glass film to be shaped to an over cladding layer. Finally, thin-film heaters of the thermooptic phase shifters and wiring for supplying power to them are formed, thereby fabricating an optical switch chip. The optical switch module is completed by connecting power supply lines and optical fibers to the optical switch chip, and by packing it into a case with a radiator fin.
A 1×128 optical switch fabricated by using the tree type arrangement as shown in FIG. 1 and the 2×2 optical switch units as shown in FIGS. 3A–3C can achieve superior characteristics with the average insertion loss of 3.7 dB and average ON/OFF extinction ratio of 50.8 dB (For example, refer to T. Watanabe et al., “Silica-based PLC 1×128 thermo-optic switch”, Proc. 27th ECOC'01, Tu.L.1.2, Amsterdam, 2001).
The conventional 1×N optical switch module, however, has a problem of requiring an enormous number of driving circuits of the thermooptic phase shifters, that is, the driving current supply circuits for the thin-film heaters. FIG. 4 shows the driving current supply circuits of the conventional tree type 1×8 optical switch. To control the individual 2×2 optical switch units, the power supply lines 11–14 of the driving current supply circuits, which connects to the 2×2 optical switch units individually, are connected to analog adjustable driving power supply circuits (not shown in FIG. 4), and the driving current supply circuits are connected to control lines. The power is supplied to one of the two thin-film heaters. The control lines are not shown in FIG. 4 to simplify the drawing.
As for the 1×N optical switch, the number of the driving power supply circuits required for the tree type construction is given by the following expression.2(log2N×1)−1
In contrast, the tap type construction requires 2N driving power supply circuits. Accordingly, as for the 1×128 optical switch, the tree type construction requires as many as 255 driving power supply circuits, and the tap type construction requires 256 driving power supply circuits.
So far, the problem is described in that the number of the driving power supply circuits is great.
Next, a problem will be described in that the area of the electrical wiring region in the PLC substrate is increasing for the above reason, and that the number of wires between the PLC substrate and driving IC assembly substrate is large.
FIG. 5 shows a conventional example of the optical switch module. The upper half of FIG. 5 shows a substrate of a 1×128 optical switch 501 using a thermooptic effect of a silica-based planar lightwave circuit (PLC), and the lower half of FIG. 5 shows an electrical wiring substrate 521 on which ICs (integrated circuits) 525 for driving the optical switch are mounted. They together constitute the 1×128 optical switch module.
As for the PLC substrate, a plurality of 1×2 optical switches 503, each of which consists of a 2-input 2-output optical switch unit, are connected in a 7-stage tree, thereby configuring a 1×128 optical matrix switch on the same substrate as shown in FIG. 5, (see T. Watanabe et. al., “Silica-based PLC 1×128 thermo-optic switch”, Proc. 27th ECOC' 01, Tu.L.1, 2, Amsterdam, 2001).
Each 1×2 optical switch 503 of FIG. 5 uses the Mach-Zehnder interferometer type 2×2 optical switch (MZI optical switch) as described above in connection with FIGS. 3A–3C.
Arranging the basic optical switches as shown in FIGS. 3A–3C in a tree construction as shown in FIG. 5 can implement the 1×128 optical switch 501. The eighth stage of FIG. 5 is added to improve the extinction ratio by gate optical switches 505.
Each gate optical switch 505 consists of the asymmetric type MZI optical switch as described above.
FIG. 6 shows an arrangement of electrical driving circuits for heaters 1101a and 1101b shown in FIGS. 3A and 3C. As shown in FIG. 6, the heaters on one-side arms of the MZIs of the 1×2 optical switches 503 and 503 are connected to the driving analog power supply circuits 31 and 32 which are adjusted such that they each supply an optimum voltage (current) for driving the MZIs. On the opposite ends of the heaters, electrical digital switches 41 and 42 are connected which are brought into conduction or out of conduction so as to turn on or off the optical switches 503 and 503. Since the total of 255 1×2 optical switches is present, there are 255 driving power supply circuits and 255 electrical digital switches as well.
An actually fabricated 1×128 optical switch can achieve excellent characteristics of the average insertion loss of 0.4 dB, and the average on/off extinction ratio of 40 dB.
Although the foregoing description is made by way of example of an optical switch, a variable optical attenuator can be constructed in the same manner by using the same MZI optical switches and by varying the variation of the phase in an analog fashion. For example, the variable optical attenuator is actually fabricated using the PLC. The variable optical attenuator is an essential device for equalizing the light intensity of the individual wavelengths of the signal light passing through the wavelength division multiplexing, and its demand has been increasing recently.
Other optical circuits growing in demand such as a dispersion compensator, polarization mode dispersion compensator and gain equalizer can also be implemented by using the MZI optical switches and by combining phase shifters and/or optical waveguides.
However, the foregoing 1×128 PLC thermooptic switch (PLC-TOSW) as shown in FIG. 5 has the following problems.
(1) The area of the electrical wiring region in the PLC substrate is large scale and apt to increase.
(2) It requires a great number of analog power supply circuits for driving, such as 255 power supply circuits in the foregoing example.
(3) The number of wire bondings between the PLC substrate and driving IC assembly substrate is increasing. The example of FIG. 5 requires 255 electrode pads 511 and 515, and 255 wiring electrode pads for connecting to the upper driving circuits, thereby requiring the total of 510 electrode pads.
(4) The inspection process requires a probe with a considerable number of pins, which in turn requires high precision probe aligning equipment.
The foregoing problems will be described in more detail one by one.
(1) There are 255 thermooptic phase shifters (heaters) to be driven on the substrate of FIG. 5, and their both ends are connected to the wiring electrode pads 511 for electrical wires at the edge of the substrate via the gold electrical circuits 507 and 509. Thus, it is necessary to layout as many as 255 gold electrical circuits 507 and 509 on the substrate without an intersection. Accordingly, the area of the electrical wiring sharply increases with the large scale of the optical circuit and the multichannels.
Next, the area of the electrical wiring will be estimated quantitatively. As for gold electrical circuits for driving the Ta2N film heaters, considering that they are patterned on the optical waveguide substrate with a bend or warp, it is preferable that they consist of electric wiring in a single layer of the a gold thin film. In addition, considering the amount of the current required for driving the heaters, it is preferable that their width is about 50 μm, and the each gap between the wires is about 50 μm. Estimating the area necessary to develop the electrical wiring under the conditions, the 510 wires require the wiring width of 51.2 mm. Since the size of the substrate is 60 mm×60 mm in the example of FIG. 5, and the average wiring length is estimated to be 4 cm when the wiring is pulled to the edge of the substrate, the area of the electrical wiring region from the heaters to the digital switches is estimated to be 20 cm2. On the other hand, the area of the optical circuit itself is reduced to 20 mm×60 mm=12 cm2 by using the core with a small permissible bending radius suitable for miniaturization and the cladding with a relative refractive index difference of 1.5%. Since the crossing layout of the optical circuit with the electrical wiring itself is possible, it can be said that the miniaturization of the PLC optical switch substrate is limited by the area of the electrical wiring region.
(2) The example of the 1×128 PLC thermooptic switch as shown in FIG. 5 has on the substrate the 255 thermooptic phase shifters (heaters) to be driven. Accordingly, it also requires as many as 255 driving (power supply) circuits.
(3) In the foregoing (1), the 510 wires on the PLC substrate must be connected one by one to the driving circuits 525 assembled on another substrate. The connection between the substrates is usually performed by wire bonding. Assume that the wire bonding electrodes 511 and 515 are 150 μm wide and the gap is 50 μm, then the pitch becomes 200 μm. Thus, the 510 wire bondings become as much as 104 mm wide.
Here, the following methods are taken to fix the PLC substrate on the electronic circuit assembly substrate: a method of pasting the PLC substrate to a larger substrate including the electronic circuit; or a method of placing the PLC substrate and electronic circuit substrate on a third substrate. Employing either method, which carries out the wire bonding at 200 degree Celsius, for example, can bring about the difference in the contraction because of the thermal expansion coefficient difference between each substrates, which can cause the stress to be imposed on wires 513, which can reduce the reliability.
(4) Furthermore, in the process of actually fabricating the module, an inspection process is essential which evaluates the optical characteristics and electrical characteristics by bringing an electrical probe into contact with the electrode pads from the outside and by driving the heaters of the PLC substrate. To conduct the evaluation, it is necessary for the conventional example to bring the 510 electrode pads into contact with the electrical probe simultaneously, which requires a special and expensive electrical probe, and aligning equipment that enables the electrical probe to make contact with the electrode pads on the PLC substrate in parallel at high precision.
Thus far, problems of the optical module are described which electrically connects the driving electrical circuit with the optical circuit by way of example of the 1×128 optical switch. These problems occur because the optical circuit and electrical circuit, which increase their size and the multichannel or multiport recently, are not optimized in their entirety. For example, as for the example described above, the problems result from the fact that the electrical circuit is not optimized even though many heaters, which are present on the optical waveguide substrate to be driven by the electrical circuit, are placed in distributed locations because of typical circumstances of the optical circuit. Similar conditions apply to optical circuits other than the 1×N optical switch, thereby constituting common problems to optical modules that electrically control the output characteristics. For example, similar problems can occur in an N×N matrix optical switch, a variable optical attenuator and its arrayed module, a dispersion compensator, and a gain equalizer.